The present invention relates to a method and apparatus for receiving an amplitude modulated (AM) radio signal, and in particular, for receiving an AM radio signal which may be transmitted in any one of a plurality of frequency channels within a predetermined frequency bandwidth.
Vehicle security apparatus is commonly operated remotely by a radio signal, which typically is transmitted by a hand-held transmitter. Such transmitters, are relatively small and are suitable for mounting on a key ring, and typically, are referred to as key fob transmitters. Alternatively, the transmitter may be contained in the head of a key. Such security apparatus typically comprises a central control circuit which receives the radio signal from the transmitter through a radio receiver associated with the central control circuit. Signals received by the radio receiver are analysed in the central control circuit to ascertain if a signal is a valid signal. On reception of a valid signal the central control circuit changes the state of the security apparatus. For example, the central control circuit may control a vehicle immobiliser for immobilising the vehicle, the central locking system for locking the doors of the vehicle, a vehicle alarm and the like. Thus, if the security apparatus is in an armed state, whereby the immobiliser is in a state for immobilising the vehicle, the central locking system is in a state with the doors locked, and the alarm circuit is armed for detecting unauthorised entry or an attempted unauthorised entry to the vehicle, on reception of a valid signal the central control circuit disarms the security apparatus thereby operating the central locking system for unlocking the doors, operating the immobiliser to allow the vehicle to be mobilised, and for disarming the alarm circuit. On the security apparatus being in the disarmed state, a valid signal received by the radio receiver causes the central control circuit to arm the security apparatus.
Typically, the remote transmitter in each transmission transmits a series of encoded signals, each of which comprises a preamble code which is common to a particular type of security apparatus, and an ID code which is specific and unique to the specific security apparatus which is to receive the signal. Typically, one to five complete signals are transmitted in each series.
It is not unusual to encounter areas in which certain frequency channels in which the signal is to be transmitted contain a relatively high amount of noise. To avoid such noisy channels it is desirable that the channel frequency at which a signal from a remote transmitter is being transmitted is varied from transmission to transmission. Additionally, it is desirable to vary the channel frequency at which the transmissions are made from transmission to transmission to minimise the danger of a signal which is transmitted by the remote transmitter being picked up and retransmitted by an unauthorised person, for example, a car thief. The channel frequency at which the transmissions are made may be sequentially or randomly varied, but more typically, are randomly varied.
In cases where the signal being transmitted is a frequency modulated signal, in general, there is little difficulty in providing a radio receiver for locating in the vehicle which is readily capable of scanning the predetermined frequency bandwidth containing the channels in which the transmissions may be made, and then locking onto a frequency of the channel at which a signal has been detected. However, in many cases it is desirable that the signal transmitted by a remote transmitter should be an AM radio signal. Unfortunately, due to the lack of suitable scanning methods and apparatus for automatically scanning a frequency bandwidth for an AM signal, it has been necessary to set the transmitter and receiver of vehicle security apparatus to transmit and receive on one single fixed AM frequency. This is undesirable.
There is therefore a need for a method and apparatus for receiving an AM radio signal which may be transmitted in any one of a plurality of frequency channels within a predetermined frequency band.